Measurements and CFX-simulations of a bubbly flow in a vertical pipe

In the Forschungszentrum Rossendorf a measurements techniques test loop was constructed. Air water flow in a vertical tube was investigated using different two phase flow measuring techniques. Air was injected into an upward water flow at normally conditions applying different injection modes. Different tests with different relations of air and water superficial velocity were performed. For each test stationary conditions were settled. The paper describes the measuring techniques used and some experiments performed. Applying a wire mesh sensor, developed in FZR, the cross section of the void fraction could be determined. The time resolution achieved by the signal processing unit is 1024 frames per second. The spatial resolution equals 3 mm. At the investigated flow velocities, the bubble diameter distributions could be determined. Using the code CFX-4.2, void profiles over the tube cross section were calculated. The development of the void profiles is mainly influenced by bubble forces, which act perpendicular to the flow direction. Therefore the comparison of the calculated with the measured results for bubble flow regimes is a good validity test for the implemented two phase flow models. The two phase models of the code version CFX-4.2 are able to describe void profiles with a near wall void maximum, which will be found for bubbly flow showing a monodisperse bubble size distribution. To model flow regimes showing a bubble size distribution and the occurrence of bubble coalescence and diffraction, model extensions would be necessary. The consideration of larger bubbles, which may be deformed, requires the development of improved bubble force models.